A manhole cover in the street becomes the lock through which a man with no name on the run from the police uses his body as the key to finding safe haven. The cover is heavy and the water below is swift and deep. The anxiety engendered by the known quantity of authority outweighs that of the fear of the unknown below and so he quickly makes his decision. A struggle with nature ensues in which the charge of water nearly takes him under and blows out the light of life burning within but he finally gains his footing and sets upon an exploration of the subterranean labyrinth which has become his sanctuary.
Recognizing the danger of remaining in the tunnel, the man is pushed to continue his exploration by some irrational impulse that almost physically exerts pressure on him to remain. Following a sound that provides no specific origin he can identify, the man arrives at place within the tunnel that gives him more room to stretch and provides a fresher quality of air. Nevertheless, the dank darkness remains and he must engage the use of a pole to continually test the depth of the water ahead. Only the occasional lighting of matches he has with him provides illumination. Finally, he reaches a point at which he can identify a very definite sound: people singing hymns. He uses pipes to climb to a position where can spy through a small crack and peer in at the sight of a group of black people dressed in white robes attending church services. Their desire for forgiveness appears to him at that moment as an obscene abomination.
Before finding a secluded spot to sleep, the man witnesses a ghastly sight: a dead infant floating lifelessly through the water. When it gets wedged between garbage, he kicks the body free with his foot and watches as it gets swept by the current. He awakens feeling cold and needing food and secure in the knowledge that his best bet is to get out of the underground sewer, but realizing that the signed confession in the hands of the cops provides no guarantee of safety above. Meanwhile, bricks he has loosened through poking at a wall with a pip reveal the office of an undertaker and he spies a corpse on a table waiting preparation for burial. A handy crowbar and tools pinched from the undertaker’s office allows the man begin prying openings into hidden passageways.
His journey through the underworld takes him to a sink where he can find clean water, jewelry shop and even a safe filled with money and gems that he is determined to keep a close on so he can learn the combination. The adventure continues into a food market offering more sustenance and clean water. While inside the market, a couple show up, thinking he works there. Strangely, although he knows he must be a sight after spending so much time in the sewers, the couple don’t seem to notice anything unusual about him. They leave and he follows where he spots a newspaper headline identifying him which sends him running back into the tunnels in fear.
He manages to find his way back to the safe and steals money and jewels as the night watchman falls asleep. Discovering a typewriter, he attempt to type out a name: freddaniels. Adding the typewriter to his stolen goods, he returns back to the little cave in the tunnels he has come to call home. An electric light and a radio taken from other basements he has wedged his way into even makes it feel more like a home. As he litters the floor with diamonds, he rationalizes away the thefts on the basis of them having no real meaning to him as possessions.
Another round of exploration in search of basements he can enter draws him back toward the sound of gospel singing again. At one point, he spies on the sight of a young boy taking a whipping for stealing a radio; the very radio that had been sitting inside his cave. Later he sees the night watchman get interrogated by the police for stealing the contents of the safe. Daniels recognizes the cops as the very ones that coerced him into signing a confession even though he did not do the crime. The watchman refuses to confess, but is determined to be guilty by the police nonetheless on the basis of his later hanging himself.
Daniels, the man who lived underground, reaches a moment of epiphany in the form of realizing that guilt is a shared quality of the collective of humanity. Taking his place back among the surface, he is determined to share this insight. He locates the police officers who got the confession from him and begs them to follow him back to his underground lair. He learns that the real culprit in the killing he was forced to confess to having committed has been identified. The police burn his written confession and dismiss him as being of no further use as he makes his way back to his subterranean home. Before he can get back to his cave, however, the cops shoot him. Just like the dead body of the discarded baby before him, the lifeless body of Fred Daniels is soon swept away by the swift current of the sewer water.